*Update* EG4 Wall Mount battery cell not reaching absorb voltage.

[IAm_Not_Lost]

I’ve had the EG4 280ah Wall Mount batteries for about six months now and they’ve worked well, but over the last week I’ve noticed one of the cells in one of my three batteries isn’t reaching full charge voltage and seems to hang a bit lower than the other cells at float voltage.

My system uses Midnite Classics for charging. Absorb voltage is set at 56.2 per the EG4 manual, and absorb time is set for 15 minutes (at the recommendation of Midnite Solar).

The EG4 BMS only appears to keep the batteries at around 56 volts during absorb for approx 8-10 minutes before it begins lessening the charge, which is before the Midnite Classis has stopped its absorb charge, so it seems like the 15 minute time is adequate.

For a week now, cell “15” will reach about 3.370 volts during charging, but generally won’t go above that. Once the charge resorts to float (set at 54 volts), it will hang around 3.340, while the other cells are around 3.37. Once the batteries have discharged some in the evening and sunset has occurred the cells will begin dropping a bit, and by around 3.310 volts all the cells will be about the same. In the morning cell “15” isn’t generally lower than the others.

My batteries generally get discharged to about 60-70% since it isn’t really too hot or cold right now. Come mid summer I expect a 40-50% depth of discharge. Currently they are hitting absorb by the afternoon and float until evening.

Anyone with insights on my next steps, or is this normal behavior and it takes a few cycles to correct itself? I am new to lithiums in this context, and how their BMS’s are programmed to operate.

Appreciate the help!

 

[shvm]

A cell low while others are high indicates top balancing issue.

For a week now, cell “15” will reach about 3.370 volts during charging, but generally won’t go above that.

There might be balancing happening. However, it may appear that the cell voltage hasn't 'moved' because of imbalance. It may take a few cycles, it may take even more than that.
It is so hard to tell because there is a lack of data from the BMS.
I expect BMS to tell, reliably, when they're balancing, what cell and with what current.

I am new to lithiums in this context, and how their BMS’s are programmed to operate.

Don't worry too much. BMS's are still way behind when it comes to proper balancing logic. I have yet to come across a BMS with a properly implemented logic.

 

[IAm_Not_Lost]

A cell low while others are high indicates top balancing issue.

There might be balancing happening. However, it may appear that the cell voltage hasn't 'moved' because of imbalance. It may take a few cycles, it may take even more than that.
It is so hard to tell because there is a lack of data from the BMS.
I expect BMS to tell, reliably, when they're balancing, what cell and with what current.


Don't worry too much. BMS's are still way behind when it comes to proper balancing logic. I have yet to come across a BMS with a properly implemented logic.

Should I increase my absorb time? I see wildly different recommendations on that.

 

[wpns]

What BMS communications do you have? I can tell when each cell in each of my six batteries is in balancing mode, so it’s in there somewhere.

 

[IAm_Not_Lost]

What BMS communications do you have? I can tell when each cell in each of my six batteries is in balancing mode, so it’s in there somewhere.

I have the basic lcd interface on the EG4 batteries, which only shows me the individual cell voltages and temperature. Along with state of charge and amps in or out. It has a few other menu’s for setting up closed loop communication, but that’s about it as far as I can tell.

It’s an open loop system using SMA sunny island inverters and midnite classic charge controllers.

I just got off the phone with EG4 and they told me to try increasing to 30 minute absorb and 57 volt charge for absorb for the next few cycles.

 

[IAm_Not_Lost]

So, the one cell in one of my batteries continues to not reach absorb charge of 3.45 or better. Only reaching about 3.37 and then charge goes to float will slowly decrease to around 3.34 and then as cells all decrease in the evening then everything generally levels out.

I did what the EG4 tech said and increased absorb voltage and time, but that did not change anything.

Anyone have any suggestions?

 

[shvm]

It means it is 'lagging behind' on charge and has to catch up. One way or the other. If increasing absorb voltage and time won't do it with time then manual intervention is required.

 

[IAm_Not_Lost]

It means it is 'lagging behind' on charge and has to catch up. One way or the other. If increasing absorb voltage and time won't do it with time then manual intervention is required.

Ok. What’s the method for manually doing it on a battery like the EG4 wallmount? Just disassembling and hooking up a charger directly to the cell?

What’s a typically timeframe to wait and let a cell “catch up”? I’m new to lithium batteries with BMS monitors.

 

[shvm]

Just disassembling and hooking up a charger directly to the cell?

yep. full disassembly isn't required. just cell terminals need to be exposed somehow. Can be even done from the balance leads connector if one really know s what they're doing.

What’s a typically timeframe to wait and let a cell “catch up”?

In my case, with a 0.6A balance current, took me almost a full week. Scale accordingly. For passive balance BMSs, the balance current is usually 150 to 200 mA

 

[IAm_Not_Lost]

yep. full disassembly isn't required. just cell terminals need to be exposed somehow. Can be even done from the balance leads connector if one really know s what they're doing.


In my case, with a 0.6A balance current, took me almost a full week. Scale accordingly. For passive balance BMSs, the balance current is usually 150 to 200 mA

If you have a moment, check out Will’s video on the EG4 battery, skip to about 6:11 and that will show you the insides. I believe he notes the cells are bolted and spot welded in series.

So, if my battery doesn’t correct itself on its own, and I decide to balance manually, would I just attach a nominal 48 volt charger to the main terminals coming off the cells inside the battery? Slowly bringing all the cells up to full charge with a small amount of amps? Or would I use a 3.6 volt charger and hook that directly to the faulty cell (wouldn’t work if I couldn’t remove the plates connecting them in series right?)?

Thanks, sorry for the numerous questions.

 

[wpns]

So, if my battery doesn’t correct itself on its own, and I decide to balance manually, would I just attach a nominal 48 volt charger to the main terminals coming off the cells inside the battery? Slowly bringing all the cells up to full charge with a small amount of amps?

Never do this. You’ll get distracted and have no COVP.

Or would I use a 3.6 volt charger and hook that directly to the faulty cell (wouldn’t work if I couldn’t remove the plates connecting them in series right?)?

Definitely do this. If there are no other connections to the battery you can charge any one cell in isolation.

 

[IAm_Not_Lost]

Never do this. You’ll get distracted and have no COVP.

Definitely do this. If there are no other connections to the battery you can charge any one cell in isolation.

Ok, so even though the low charge cell is still connected in series, I could charge that in isolation if no load is on the battery using a 3.6 volt charger? Any recommendations on a budget charger to do that?

 

[TM48]

I don't see you having a problem. I have three wall mounts (48 cells) which depending on the charge/balancing state can vary up to 62 millivolts. It's highest when only balancing is active. I typically see ~40 millivolts spread during heavy charge. When the SCC's shut down for the night the cells rapidly converge to within 3-4 millivolts. Manufacturing tolerances will cause some variation between the charge/discharge envelopes of each battery. As long as one cell doesn't trend away from the rest and not return I wouldn't worry about it. BTW the BMS doesn't control the absorb voltage the SCC does.BMS's typically set protection limits only. The BMS in the wall mounts does set the max current range depending on the SOC. The charge controller is what sets the charge profile.

 

[TM48]

I run closed loop with two 6000xp's. Here is a snap shot of my batteries from Solar Assistant. Note the highest cell voltage. It's only balancing right now.

 

[Koyaanisqatsi]

As long as one cell doesn't trend away from the rest and not return I wouldn't worry about it.

This is exactly what is happening. The OP has run this system at recommended settings and the one cell is drifting lower and lower in voltage over charge cycles.

I'm working on this same problem with a friend of mine with 4 x EG4 rack batteries. The general solution to cells falling out of balance with everything configured to spec is to raise the charge voltage and hold the absorb for longer. The BMS might behave in certain ways that prevent continued charge current after a certain point (mine does this). But don't let the voltage drop to the float value until you can confirm that all cell balancing activity has stopped. If you can't see the balancing activity, then let it sit at absorb voltage for at least a couple-few hours per day or charge cycle.

 

[TM48]

Once the batteries have discharged some in the evening and sunset has occurred the cells will begin dropping a bit, and by around 3.310 volts all the cells will be about the same. In the morning cell “15” isn’t generally lower than the others.

Doesn't appear to be

 

[IAm_Not_Lost]

Doesn't appear to be

So, this cell hasn’t been doing this until recently. I wouldn’t say it’s gotten significantly worse, but it’s about 40mv less at float, and about 60mv less at absorb charge than the other cells. I have increased my absorb voltage a little and increased to 30 minutes and haven’t seen any difference. The BMS still cuts off the absorb charge after about 8 minutes and then it slowly reduces charge down to float.

 

[TM48]

At that time what is the highest cell?

 

[IAm_Not_Lost]

At that time what is the highest cell?

It’s been a minute since I’ve been able to be home as it’s hitting absorb, but generally all the other cells have reached their appropriate absorb voltage. I don’t remember the exact exact number.

 

[fnnwizard]

So, this cell hasn’t been doing this until recently. I wouldn’t say it’s gotten significantly worse, but it’s about 40mv less at float, and about 60mv less at absorb charge than the other cells. I have increased my absorb voltage a little and increased to 30 minutes and haven’t seen any difference. The BMS still cuts off the absorb charge after about 8 minutes and then it slowly reduces charge down to float.

Greater than 10mV cell imbalance below 3.400V is definitely not ideal.

I'm not sure how the bms of the wall mount terminates V when running under soc or what's its balancing parameters are since many revisions to that has taken place.

If you can change it, make it start to balance at 3.41V with a diff of 10-15mV. This should help to keep in in balance at lowest V possible.
If you do not open the case to individually charge/discharge cells. You may have to float at a higher V to help the bms balance the cells. How high depends on the bms balancing parameters.

This post below should give you insight to termination V wrt current and vice versa.

F

Post in thread 'What is the accurate LifePO4 resting voltage vs SOC chart'

Jun 3, 2025

I have put together a Seplos battery kit with 16x EVE 280AH grade A batteries but I don't think I am using the full capacity. I have no solar so the charge is up to 7 hours at night from the grid. You can see from last night's results the charge is 37.9A. The charge terminates at 54.5v which I thought was 85%. I see the voltage drop (overpotential?) to 53.4v over an hour at zero current. This seems to imply the cells are at 77%. From the previous comments, should I increase the charge cut-off such that after an hour I get 3.43v (54.88V) and reset the BMS to call that higher charge...

• fnnwizard

 

[IAm_Not_Lost]

Greater than 10mV cell imbalance below 3.400V is definitely not ideal.

I'm not sure how the bms of the wall mount terminates V when running under soc or what's its balancing parameters are since many revisions to that has taken place.

If you can change it, make it start to balance at 3.41V with a diff of 10-15mV. This should help to keep in in balance at lowest V possible.
If you do not open the case to individually charge/discharge cells. You may have to float at a higher V to help the bms balance the cells. How high depends on the bms balancing parameters.

This post below should give you insight to termination V wrt current and vice versa.

F

Post in thread 'What is the accurate LifePO4 resting voltage vs SOC chart'

Jun 3, 2025

I have put together a Seplos battery kit with 16x EVE 280AH grade A batteries but I don't think I am using the full capacity. I have no solar so the charge is up to 7 hours at night from the grid. You can see from last night's results the charge is 37.9A. The charge terminates at 54.5v which I thought was 85%. I see the voltage drop (overpotential?) to 53.4v over an hour at zero current. This seems to imply the cells are at 77%. From the previous comments, should I increase the charge cut-off such that after an hour I get 3.43v (54.88V) and reset the BMS to call that higher charge...

• fnnwizard

As far as I am aware I can’t change the BMS parameters. I have EG4 Wall mounts with SMA sunny island inverters and the batteries are charged by Midnite classics. Open loop.

Could I lower the absorb charge to just under the cut off for the BMS so that it sits there charging the batteries for longer? Basically play around and set the Classics to deliver just enough charge so that the BMS still keeps allowing it and slowly bring the low cell up, but not enough voltage so that the BMS cuts off the absorb voltage since the remaining cells have reached charge?

Like if I set the Classics to deliver like 55.2 volts or thereabouts? And just let it do that for an hour or two for a couple of days? And then set it back to 56.2 volts like EG4 recommends to get the normal absorb charge.

 

[fnnwizard]

As far as I am aware I can’t change the BMS parameters. I have EG4 Wall mounts with SMA sunny island inverters and the batteries are charged by Midnite classics. Open loop.

Could I lower the absorb charge to just under the cut off for the BMS so that it sits there charging the batteries for longer? Basically play around and set the Classics to deliver just enough charge so that the BMS still keeps allowing it and slowly bring the low cell up, but not enough voltage so that the BMS cuts off the absorb voltage since the remaining cells have reached charge?

Like if I set the Classics to deliver like 55.2 volts or thereabouts? And just let it do that for an hour or two for a couple of days? And then set it back to 56.2 volts like EG4 recommends to get the normal absorb charge.

You should be able to change the parameters with their BMS tools.
If you don't, then you would have to hold the V at a higher level.

To figure out how high to start the float, bulk/absord charge until any cell hits 3.55V max and current is around 8A. Note which cells and note total V.

Next float at 54.4V to begin, you'll eventually need to raise this float V to what ever V is from previous bulk/abosrb V. So, yes you can increase float V to balance the cells, but you have to be carful not to let the higher cells sit at too high of a V at the lower float current for too long.

The BMS parameters if they balance to within 20mV makes you use a high float V for proper balance since the higher you go the more dif the cells will show. Balancing to 20mV at 3.45V is about good enough if current ~ 3A for the 280cells. But the balancing resistors discharging at 150mA or so will allow the total V to get too far and all charging stops again.

The only way to truly balance wtihout "over charging" is with a "balancing charger" like in this picture. Here, I balance at 3.55V per cell at 10A but with all cells within 1mV of each other. You'll also see I still need to manually discharge cells to help the charger out if cells are too much out of balance.

 

[IAm_Not_Lost]

So, before doing anything too major on my own I reached out again to Signature Solar as the issue is slowly getting worse, and actually got them to talk to their “advanced tech”, and they want me to force the batteries into “over-volt” by setting the absorb on my charge controllers to 58v.

The BMS should then be triggered into top balancing. They state it won’t cause harm and the BMS should recognize and halt charging for protection. So, we’ll see. They couldn’t give me a defined time to set absorb, sounds like just long enough for the “over-volt” to be triggered.

Thoughts?

 

[On_The_Road]

EG4 ships BMSs with passive balancers. How is a 100ma or less balancer supposed to take care of a pack that has 280,000mah of capacity?

 

[wpns]

EG4 ships BMSs with passive balancers. How is a 100ma or less balancer supposed to take care of a pack that has 280,000mah of capacity?

Very slowly, but they don't need a lot. (Almost) every day I'm at 100% for a few hours, and my batteries balance for some random time:
EG4_PP_BatteryBalanceMinutes (Backwards from last night):
[0, 74, 150, 63, 75, 69]
[0, 59, 100, 33, 36, 77]
[0, 0, 33, 0, 0, 22]
[0, 0, 0, 0, 0, 0] <--- Didn't hit 100%
[0, 22, 27, 67, 13, 29]
[0, 27, 16, 14, 5, 87]

Ignore the zeros on the first battery, it's using the Secret Battery One Protocol, but I imagine it's somewhere in the range of the others..

So sometimes just a few minutes, sometimes a couple of hours, but they always seem to be under 40mv Delta pretty quickly.

40mv is the balance threshold, so I'd guess I'm OK.